Ceramic ferroelectric composite material - BSTO-MgO

ABSTRACT

A novel ceramic ferroelectric material having a low dielectric constant, extremely low loss and high tunability. The material is a composite comprising Barium Strontium Titanate (BSTO) and a ceramic material having a low dielectric constant. The preferred composite is represented by Ba 1-x  Sr x  TiO 3  --MgO, wherein x is greater than 0.00, but less than or equal to 0.75, and wherein the percent weight ratio between Ba 1-x  Sr x  TiO 3  and MgO ranges from approximately 99%-40% and 1%-60%, respectively. The novel materials possess superior electronic properties; and they may be employed in various antenna systems at both microwave and millimeter wave range frequencies.

GOVERNMENTAL INTEREST

The invention described herein may be manufactured, used and licensed byor for the U.S. Government without payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 08/076,291, filed on Jun. 9, 1993, now U.S. Pat.No. 5,312,790. It is also copending with related U.S. patentapplications Ser. Nos. 08/207,447 (entitled "Novel Ceramic FerroelectricComposite Material--BSTO--ZrO₂ "), and 08/215,877 (entitled "NovelCeramic Ferroelectrics for Phased Array Antennas"). These patentapplications are commonly owned by the U.S. Government as represented bythe Secretary of the Army.

The need exists for the fabrication of ceramic materials having improvedelectronic properties which may be adjusted for a particular, intendeduse. The present invention deals with novel ceramic ferroelectricmaterials having ideal properties for use, for example, in phased arrayantenna systems.

The ferroelectric materials are a replacement for the more expensivecurrent driven ferrites which are currently used in phased arrayantennas. The invention outlines ferroelectric materials which provideadequate phase shift with a minimal insertion loss.

Current attempts to use ferroelectric materials employ porous ceramics,whose properties are less than ideal for their intended application.Porous ceramics of the Ba_(1-x) Sr_(x) TiO₃ type are commonly employedin ceramic phase shifter antennas. However, these materials displaycertain deficiencies due to both the processing difficulties andexpense, as well as their overall electronic and microwave properties.These deficiencies include electronic inhomogeneity, structuralweakness, reproducibility and processing control, and large losstangents.

Barium Strontium Titanate (BaTio₃ --SrTiO₃), also referred to herein asBSTO, has been known to be used for its high dielectric constant(approximately ranging from 200 to 6,000) in various antennaapplications. This is set forth by Richard W. Babbitt et al. in theirpublication, "Planar Microwave Electro-Optic Phase Shifters," MicrowaveJournal, Volume 35(6), (June 1992). This publication concludes thatthere exists a need for additional research to be conducted in thematerials art to yield materials having more desirable electronicproperties.

Although the employ of BSTO in phase shifters is known, nowhere in thetechnical arena of ceramic art has there been any suggestion ofmodifying BSTO, or combining BSTO with additives, in the mannerdescribed herein. Moreover, the specific BSTO combinations, which haveenhanced electronic properties, are deemed novel.

The present invention provides for improved materials which exhibitelectronic properties which can be adjusted for use, for example, in anydiscrete element phase shifter design--i.e. planar microstrip, waveguide geometries or parallel plate structure. The invention herein issuperior to other currently used ferroelectric materials in the art. Theinvention is designed to provide tunable materials with an extremelyminimal insertion loss. Therefore, these materials can be used inmillimeter wave applications with 77 GHz frequency range.

BRIEF DESCRIPTION OF THE INVENTION

The subject matter of the present invention relates to the fabricationof specific ceramic materials which have sought after properties in, forexample, phased array antenna systems. The sought after propertiesinclude (1) a moderate dielectric constant; (2) a low loss; and (3) hightunability. Dielectric constant is related to the energy storage in thematerial; whereas, the loss tangent is related to the power dissipationin the same material. In general, the dielectric function is a complexquantity with ε=ε'-iε"; and the loss tangent, tan δ=ε"/ε'=0.001 or less.

Tunability may be defined as ((dielectric constant with no appliedvoltage)-(dielectric constant with an applied voltage))/(dielectricconstant with no applied voltage). For simplicity purposes, tunabilitycan be represented as T ##EQU1## wherein, X=(dielectric constant with noapplied voltage); and

Y=(dielectric constant with an applied voltage).

The tunability of a material under an electric field of 7.0 KV/cm canrange from 1-60% depending upon the composition of the materialsemployed.

The materials herein combine Barium Strontium Titanate (BaTiO₃ --SrTiO₃)with Magnesium Oxide (MgO). These materials, encompassed by the presentinvention, are superior in that they are homogeneous, extremely dense,easily machinable, and possess superior electronic properties at both dcand microwave operating frequencies. Moreover, the materials herein havelow water absorptivity. Typically these materials will absorb less than2% by weight of water therein. Hence, the materials within the scope ofthe present invention are environmentally stable--for example, they havegood moisture and temperature stability.

Although other combinations of electrically active and inactivecomponents have been commonly employed in conjunction with piezoelectricmaterials, nowhere has the combination of the present invention beendescribed. More specifically, the present invention is the firstteaching wherein BSTO is combined with magnesia in order to adjust theelectronic properties and phase shifting ability of a material.Specifically, nowhere has BSTO been combined with magnesia to adjust theelectronic properties of the material for use in a phase array antennasystem. Aside from the combination of BSTO with magnesia being novel,its application in phase array antenna systems is an application neversuggested in the prior art.

Replacing the currently employed materials with the novel ferroelectriccomposite described in the present invention will improve the overallperformance of a phased array antenna system as well as reduce the cost,weight and size of the antenna per se.

Accordingly, it is an object of the present invention to provide aferroelectric material suitable for, but not limited to, use in phasedarray antenna systems.

It is a further object of the present invention to fabricate a materialexhibiting enhanced electronic properties.

It is still a further object of the present invention to provide aferroelectric material having a moderate dielectric constant, anextremely low loss and a high tunability.

It is a further object of the present invention to provide materialshaving electronic properties, wherein said electronic properties can beadjusted in such a manner that they can be employed in any discreteelement phase shifter design.

It is a further object of the present invention to provide aferroelectric material which is easily machinable.

Still, it is a further object herein to provide a ferroelectric materialwhich possesses superior electronic properties at dc, microwave andmillimeter wave operating frequencies.

The means to achieve these and other objectives of the present inventionwill be apparent from the following detailed description of theinvention and claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention encompasses the fabrication of novel ceramicmaterials having enhanced electronic properties. These materials aresuperior to other currently employed ferroelectric materials.

When one considers the optimization in the electronic properties ofceramic materials, the following parameters must be taken intoconsideration:

(1) Dielectric Constant: Ideally the dielectric constant should be low,ranging from approximately 79 to 2179 . This dielectric constant rangedoes not decrease the phase shifting ability of the material if asufficient length of material is used (then a high dielectric constantis not needed). As insertion loss (loss of energy getting into theceramic) does not depend upon the dielectric constant, it is noteffected by lowering the dielectric constant. Also, since the losstangent (tan δ) increases with increasing dielectric constant (for theseferroelectric materials), lower dielectric constant materials tend tohave lower loss tangents and therefore, less insertion loss. However,even samples of these composites of BSTO--MGo, wherein Ba=0.60, withhigher dielectric constants (above approximately 800) have low loss(double zero--less than 0.001) but decreasing ε_(r) reducing tan δ byapproximatelty 40% to tan δ=0.00.

(2) Low Loss: The loss tangent (intrinsic to the material) serves todissipate or absorb the incident microwave energy and therefore is mosteffective in this device when the loss tangent is in the range of 0.002or less. The low loss tangent serves to decrease the insertion loss andhence increase the phase shifter per decibel of loss. The operatingfrequency is controlled by the loss tangent. Extremely low lossmaterials (0.0007) can be used at millimeter wave range frequencies.

(3) High Tunability: The tunability of a particular material effects thematerial's electronic properties by how much the dielectric constantchanges with applied voltage. The amount of phase shifting ability isdirectly related to the tunability; therefore, higher tunabilities aredesired. The tunability can be increased to some extent by decreasingthe sample thickness. The insertion loss is inversely related to thetunability so that the larger the tunability, the smaller the insertionloss. Optimum electronic properties would have tunabilities ranging from7 to 36% (depending upon other factors, dielectric constant and losstangent).

The materials within the scope of the present invention fall within theoptimum characteristics outlined above. These materials are Ba_(1-x)Sr_(x) TiO₃ --MgO, wherein x is greater than 0.0 but less than or equalto 0.75. This formulation may be referred to as Barium StrontiumTitanate and magnesia. The weight ratios of Barium Strontium Titanate(BSTO) to magnesia may range from 99% wt.-40% wt. BSTO to 1% wt.-60% wt.magnesia. A typical composition within the present invention maycomprise 70% by weight BSTO (wherein x=0.35) and 30% by weight magnesia(MgO). This composition has a dielectric constant of 425.2, a losstangent of 0.0006 and a tunability of 18.00 (applied electric field=20.3KV/cm).

Magnesia is used herein to adjust the electronic properties of BSTO.Magnesia at low doping levels (1-10% wt.) lowers the Curie temperature(temperature at which the peak dielectric constant occurs). At higherlevels, it lowers the material's dielectric constant and loss to meetthe requirements for various applications--for example, in the antennaarts. The electronic properties of the formulation herein can beadjusted for use in any discrete element phase shifter design, such asplanar microstrip, wave guide geometries or for use in a parallel platestructure.

It has been found that the electronic properties of BSTO magnesia arereproducible to within 2%. Hence, once a specific formulation of BSTOmagnesia is determined to be suitable for a specific purpose, thematerial can be accurately reproduced.

The preparation of BSTO magnesia may be accomplished by obtainingpowders of Barium Titanate and Strontium Titanate. These powders areball milled in a conventional manner in an organic solvent. Thisparticular mixture is then air-dried and calcined at approximately 200degrees below the sintering temperature for several hours. The resultantBSTO is then mixed with magnesia in the desired weight percentage andre-ball milled in an organic solvent with a binder. The final mixture isthen air-dried, once again, and dry-pressed at approximately 7,000p.s.i. The final samples are sintered in air. Proper electroding of thecomposite ceramics must be done. The samples were screen printed with aFERRO #3350 (Electronic Materials Division, Santa Barbara, Calif.)silver conductive ink. They were subsequently fired at 450° for ten (10)minutes. The samples were then dipped in a bath of 2% silver (Ag), 62%tin (Sn) and 36% lead (Pb) solder with lead clips attached.

Table 1 sets forth the various properties of BSTO magnesia, wherein theformulation is represented by Ba₀.60 Sr₀.40 TiO₃ - magnesia.

                  TABLE 1                                                         ______________________________________                                        Magnesia                                                                      Content (wt. %)                                                                          Density (g/cc)                                                                            % Porosity                                                                              % Absorption                                 ______________________________________                                        1.0        5.00        10.70     1.94                                         5.0        5.300       3.97      0.63                                         10.0       5.192       3.36      0.55                                         30.0       4.689       4.27      0.81                                         60.0       3.940       2.56      0.751                                        80.0       3.5180      10.34     1.87                                         ______________________________________                                    

The electronic properties of some of the formulations within the presentinvention are set forth in Tables 2 and 3. The representativeformulations for which electronic properties are tabulated are for BSTOat Ba=0.65 and Ba=0.60 with varying magnesia content. Frequency used was1 kHz and dielectric constants have been corrected for edge (fringe)capacitance.

                  TABLE 2                                                         ______________________________________                                        BSTO (Ba = 0.65) and Magnesia                                                                                      Electric                                 Magnesia  Dielectric                                                                              Loss     Tunability                                                                            Field                                    Content wt. %                                                                           Constant  Tangent* (Percent)                                                                             (V/μm)                                ______________________________________                                        1.0       2178.97   0.00186  25.20   1.77                                     10.0      1481.30   0.00163  21.47   1.76                                     30.0      718.06    0.00112  36.26   3.72                                     60.0      79.20     0.00055  10.66   2.34                                     ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        BSTO (Ba = 0.60) and Magnesia                                                                                      Electric                                 Magnesia  Dielectric                                                                              Loss     Tunability                                                                            Field                                    Content wt. %                                                                           Constant  Tangent* (Percent)                                                                             (V/μm)                                ______________________________________                                        1.0       1047.33   0.00149  16.08   2.27                                     5.0       1566.22   0.00141  --      --                                       10.0      1167.18   0.00118  --      --                                       15.0      895.78    0.00106  7.26    1.86                                     20.0      886.45    0.00096  15.95   2.27                                     25.0      650.91    0.00076  17.46   2.40                                     30.0      433.43    0.00087  9.35    1.62                                     35.0      425.18    0.00065  18.00   2.03                                     40.0      306.92    0.00092  19.81   2.53                                     50.0      188.65    0.01176**                                                                              9.55    2.14                                     60.0      89.35     0.00065  11.09   2.63                                     ______________________________________                                         Note:                                                                         *The magnitude of the loss tangents reported approach the limit of            measurement capability of the test apparatus; therefore, actual loss          tangents are in some cases less than these values.                            **Poor contact, actual loss tangent less than above.                     

If the antenna application does not require exceedingly high tunability(where tunability can be increased with a decrease in sample thicknessfor a given externally applied electric field), then the compositionswith lower dielectric constants are probably likely to produce lessimpedance mismatch and may possess lower loss tangents.

EXAMPLE 1

Powder forms of Barium Titanate and Strontium Titanate were obtainedfrom Ferro Corp., Transelco Division, Pen Yan, N.Y. (product nos. 219-6and 218, respectively). The powders were stoichiometrically mixed in aslurry of ethanol and ball-milled using alumina 3/16" grinding media.This was performed for 24 hours. The mixture was subsequently air driedand calcined for 5 hours at approximately 1100° C. The resulting BSTOwas mixed with powder Magnesia (Johnson Malthey Electronics, Ward Hill,Mass., product number 12287) in the proper weight percent. This mixturewas then re-ball milled in a slurry of ethanol using a 3/16" aluminagrinding media for an additional 24 hours.

To the resulting BSTO/Magnesia mixture, Rhoplex B-60A (Rohm and HaasCo., Philadelphia, Pa.), which is a 3% wt. organic binder consisting ofan aqueous emulsion of acrylic polymer, was added to improve green bodystrength and to permit sample fabrication in greater dimensions. (Greenbody strength refers to the ability of unfired material to remain intactand to withstand handling; it also implies better densities in theunfired pieces.) Other binders and plasticizers may be added at thispoint to allow extrusion molding or for fabrication of tape-cast sheetsof material.

The mixture is then air-dried and dry-pressed to a pressure ofapproximately 7,000 p.s.i. Sintering schedules are ascertained byemploying a deflectometer such as a Mitutoyo digimatic indicator andminiprocessor (Mitutoyo Corp., Paramus, N.J.). The final samples werefired in various furnaces and the densities of the samples were found tobe reproducible to within 1 to 2%.

The properties of the resulting BSTO-Magnesia samples are set forth inTable 1, above.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention. For example, the invention may be modified to includeceramic-ceramic composites of BSTO and other low dielectric constantmaterials depending upon the particular requirements of the intendedapplication. Among some of the other low dielectric constant materialswhich may be combined with BSTO are zirconia, alumina microballoons,alumina fibers or fabric, silicon dioxide and other low dielectricconstant, low dielectric loss oxides. (Alumina microballoons are hollowspheres of approximately 1-5 microns in diameter and are alreadysintered components (BSTO/ceramic)--the electronic properties of acomposite employing alumina microballoons will most likely differ fromcomposites employing alumina powder. Alumina fibers or fabric, whenemployed in the composite within the scope of the present invention, maypossess electronic properties different from composites which employalumina powder. This is due to the fact that this form of alumina ismost likely to be in sintered form; and the fibers or fabric aluminaproduce different connectivity between the BSTO particles.)

It is, therefore, intended that the claims herein are to include allsuch obvious changes and modifications as fall within the true spiritand scope of this invention.

I claim:
 1. A ceramic ferroelectric composite material consistingessentially ofBarium Strontium Titanate, said Barium Strontium Titanaterepresented as Ba_(1-x) Sr_(x) TiO₃, wherein x is greater than 0.0 butless than or equal to 0.75; and magnesia; wherein said Barium StrontiumTitanate and said magnesia are present in amounts to provide a compositehaving a low dielectric constant, low loss tangent and high tunability.2. The ceramic ferroelectric composite material of claim 1, wherein saidBarium Strontium Titanate is Ba_(1-x) Sr_(x) TiO₃, wherein x=0.35 to0.40.
 3. The ceramic ferroelectric composite material of claim 2,wherein the weight ratio of said Barium Strontium Titanate to magnesiaranges from approximately 99%-40% Barium Strontium Titanate toapproximately 1%-60% magnesia.
 4. The ceramic ferroelectric compositematerial of claim 3, wherein the ratio of Barium Strontium Titanate tomagnesia is approximately 70% wt. Barium Strontium Titanate toapproximately 30% wt. magnesia.